What came to my mind is that 2 users tried to insert a row at similar times and both got the same ID from the VB function.
Why don't you have any constraint in this column?
Luis C.
General Disclaimer:
Are you seriously taking the advice and code from someone from the internet without testing it? Do you at least understand it? Or can it easily kill your server?
How to post data/code on a forum to get the best help: Option 1 / Option 2